This application relates to hard disc drives and more particularly to an apparatus and method for detecting the back electromotive force (BEMF) signal of the spindle motor and using that BEMF signal to generate a spindle motor drive signal.
In a disc drive, data is recorded on a disc in concentric, circular paths known as tracks. Servo bursts are written in each track on the disc and contain position information. During operation the disc continually rotates and a read/write head a given radius from the center of the disc reads or writes data in a given track. An actuator arm swings the head in an arc across the disc surface to allow the head to read or write data in different tracks.
The disc must rotate at a nearly constant angular velocity so that the data can properly be read from the disc. The data is written while the disc rotates at the same nearly constant angular velocity so the timing of the read process mimics the timing of the write process. To provide the nearly constant angular velocity, a spindle motor is used and usually takes the form of a direct current (DC) brushless motor. DC brushless motors often have multiple coils or phases and each phase must be energized or commutated at the appropriate time and with the appropriate current orientation and amplitude. Furthermore, the amount of voltage dropped across each coil must be tightly controlled to maintain the constant velocity.
A common mode of operating a DC brushless motor is the bipolar operating mode using stepped driving signals. In this mode for a three phase motor, at any given time two coils are energized and one coil floats. A six step drive signal is applied to each coil, two steps on, then one off, then two in the reverse, and then one back off. The timing of these signal steps must be correct for the motor to properly rotate in the desired direction and at the desired velocity.
The DC brushless motor""s coils generate a BEMF as they rotate through magnetic fields. As each phase rotates between separate stator fields, the BEMF signal behaves similarly to a sinusoid as it decreases to zero and then switches polarity and increases back to its peak. Thus, comparators can be used to determine when the BEMF signal crosses zero potential, and the stepped drive signal is timed so that the zero step occurs when the BEMF signal crosses zero. The peak value of the BEMF signal also indicates the velocity of the motor and can be sampled to maintain the proper velocity by altering the current provided to the motor in response to a BEMF peak that is too high or low. This system achieves a nearly constant velocity but the velocity modulates due to a torque ripple associated with the switching of the drive current steps.
To help minimize undesirable torque ripple, which alters the velocity of the motor, and to minimize related acoustic noise, sinusoidal drive currents are applied to the phases instead of the normal six steps. The sinusoid must properly be in phase with the motor""s rotation. Hall sensors, resolvers, and feedback encoders can be used to sense the motor""s position and to determine the timing of the application of the sinusoid. However, these devices add significant expense and bulk to the motor. Additionally, the sinusoidal drive current is usually derived from values in a lookup table that occupies valuable memory space. Although torque ripple is decreased, some still exists because the drive signals cannot account for mechanical differences between each coil in the motor that gives each coil a unique BEMF signal with harmonic distortion. The unique BEMF signal alters the voltage that must be dropped across each coil to maintain a constant velocity, and the unique BEMF signal causes each coil to respond differently to the generic drive signals being applied.
These problems are addressed by the present in methods and systems that provide autosynchronization of the drive signal with the motor""s rotational position without using position sensors. The present invention""s methods and systems also reduce velocity modulation and acoustic noise associated with torque ripple and save memory space by driving the motor""s coil with a drive signal created from amplification of the back emf signal. The drive signal is proportional to, in phase with, and at the same frequency as the back electromotive force being produced by the motor""s coil.
A method implementing the present invention drives the phase of an electrical motor by executing several steps. A back electromotive force being generated by the motor coil is detected. This back electromotive force is amplified to produce a drive signal that is fed back to the motor""s coil. This drive signal is proportional to the back electromotive force and has sufficient amplitude to also overcome the resistive losses of the motor. The drive signal is in phase with and at the same frequency as the back emf of the coil and is therefore, automatically in synchronization with the motor""s rotational position.
A control system implementing the present invention is configured to detect the back emf with a detection circuit and amplify it to overcome losses in the coil before sending it to the coil as a drive signal. An exemplary detection circuit contains several difference amplifiers that find the total voltage on the coil, the total resistive loss voltage dropped across the coil, and the difference between the total voltage and the resistive loss voltage, which is the back emf. The back emf is fed to a gain controller which produces a signal that is proportional to the back emf signal and also has an amplitude that is sufficient to produce a drive signal that overcomes the coil""s resistive losses.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.